Flue gas purifying device

ABSTRACT

A flue gas purifying device includes an exhaust pipe that guides flue gas discharged from an internal combustion engine; a catalytic unit that is arranged on a downstream side to the internal combustion engine in a flow direction of flue gas and includes a nitrogen-oxide storage-reduction catalyst that stores nitrogen oxides contained in flue gas and a support mechanism that is arranged in the exhaust pipe and supports the nitrogen-oxide storage-reduction catalyst in the exhaust pipe; a reducing-agent injecting unit that injects a reducing agent to the catalytic unit in the exhaust pipe; a concentration measuring unit that is arranged on a downstream side to the catalytic unit in the flow direction of flue gas and measures a concentration of nitrogen oxides in flue gas having passed through the nitrogen-oxide storage-reduction catalyst; and a control unit that controls whether to inject the reducing agent from the reducing-agent injecting unit based on a concentration of nitrogen oxides measured by the concentration measuring unit.

FIELD

The present invention relates to a flue gas purifying device thatreduces nitrogen oxides discharged from an internal combustion engine.

BACKGROUND

Gas discharged from an internal combustion engine such as a dieselengine, a gasoline engine, and a gas turbine, that is flue gas, containsnitrogen oxides (NOx) and particulate matters (PM). Particularly,because the diesel engine burns fuel in an oxygen excessive state, alarge amount of nitrogen oxides (NOx) and particulate matters (PM) arecontained in flue gas. Therefore, a device that decreases particulatematters or a device that decreases nitrogen oxides is provided in anexhaust pipe of an internal combustion engine. As an example of thedevice that decreases nitrogen oxides, there is a device that decreasesnitrogen oxides from flue gas by injecting urea into an exhaust pipethat guides flue gas, produces ammonia from urea in the exhaust pipe,causes the produced ammonia to react with nitrogen oxides in flue gas,and then deoxidize nitrogen oxides to produce nitrogen again.

For example, Patent Literature 1 describes a flue gas purifying devicein which a DPF device and a selective catalytic reduction catalyticdevice are sequentially arranged from an upstream side in an exhaustpath of an internal combustion engine. Patent Literature 1 alsodescribes a device that calculates NOx emissions, at the time of anormal operation, based on a NOx emissions map for the normal operation,or at the time of forced regeneration of the DPF device, calculates NOxemissions based on a NOx emissions map for the forced regeneration, tocalculate a feed rate of ammonia aqueous solution corresponding to thecalculated NOx emissions, and feeds ammonia aqueous solution into fluegas on an upstream side of the selective catalytic reduction catalyticdevice so as to reach the calculated feed rate.

Further, Patent Literature 2 describes NOx removal equipment for fluegas discharged from a combustion plant such as a waste incinerator,although it is not for treatment of flue gas from an internal combustionengine. Patent Literature 2 describes a denitration control method inwhich a NOx concentration in gas before treatment, an ammoniaconcentration in treated flue gas, a NOx concentration in flue gas, anda flow rate of flue gas are measured, to calculate a flow rate of NOxbefore treatment, a NOx concentration after treatment, a record of NOxremoval efficiency by NOx removal equipment, and an ammoniaconcentration in treated flue gas based on a measurement result thereof,deviations between the calculated values and target values thereof arerespectively calculated to thereby calculate correction values based onthe calculated deviations, and a corrected flow rate of NOx iscalculated based on at least one of the calculated correction values,thereby controlling a flow rate of ammonia to be injected into flue gasbefore treatment based on the calculated corrected flow rate of NOx.

Furthermore, as an apparatus for decreasing nitrogen oxides, there is anapparatus that stores nitrogen oxides in a nitrogen-oxidestorage-reduction catalyst, produces carbon monoxide or hydrogen byinjecting light gas oil in a tube or a pipe with a predeterminedinterval and by partially oxidizing the light gas oil, and reduces thenitrogen oxides stored in a storage catalyst by the produced carbonmonoxide or oxygen.

CITATION LIST Patent Literatures

Patent Literature 1: Japanese Patent Application Laid-open No.2007-154849

Patent Literature 2: Japanese Patent Application Laid-open No.2005-169331

SUMMARY Technical Problem

In a flue gas purifying device that uses a nitrogen-oxidestorage-reduction catalyst (hereinafter, referred as “NOx storagecatalyst”), there is a limit to the nitrogen oxide storage capacity of aNOx storage catalyst. Therefore, as described above, reduction treatmentneeds to be performed with a predetermined interval. The timing ofperforming the reduction treatment is calculated based on a data mapshowing a correlation of engine speed and torque with the temperature offlue gas and the concentration of nitrogen oxides in flue gas. Bycalculating a discharge amount of nitrogen oxides based on the data map,the reduction treatment can be performed before a threshold limit of thestorage capacity of the NOx storage catalyst is reached.

However, in the NOx storage catalyst, the storable amount of nitrogenoxides varies according to the temperature of the catalyst. Therefore,at the time of an unsteady operation, it is difficult to ascertain theamount that can be stored by the catalyst accurately. Further, becausethe NOx storage catalyst deteriorates due to being used, the amount thatcan be stored by the catalyst is not constant at all times. Therefore,even if the timing of the reduction treatment is controlled based on adata map, appropriate control cannot be performed at the time of anunsteady operation, or when a rapid temperature change or deteriorationof the catalyst occurs.

If a reducing agent (for example, light gas oil) is excessively input atthe time of performing the reduction treatment of the NOx storagecatalyst, reduction proceeds too much to produce ammonia. Accordingly,if the reduction treatment is performed in a state that NOx is notstored in the NOx storage catalyst, ammonia is produced. Therefore,performing the reduction treatment of the NOx storage catalyst more thannecessary without accurately ascertaining the storage capacity of theNOx storage catalyst is problematic.

The present invention has been achieved to solve the above problem, andan object of the present invention is to provide a flue gas purifyingdevice that can appropriately perform reduction treatment of a storagecatalyst, to suppress leakage of nitrogen oxides and ammonia to adownstream side, and to decrease nitrogen oxides in flue gasefficiently.

Solution to Problem

According to an aspect of the present invention, a flue gas purifyingdevice that reduces nitrogen oxides contained in flue gas dischargedfrom an internal combustion engine, the device including an exhaust pipethat guides flue gas discharged from the internal combustion engine, acatalytic unit that is arranged on a downstream side to the internalcombustion engine in a flow direction of the flue gas and includes anitrogen-oxide storage-reduction catalyst that stores nitrogen oxidescontained in the flue gas and a support mechanism that is arranged inthe exhaust pipe and supports the nitrogen-oxide storage-reductioncatalyst in the exhaust pipe, a reducing-agent injecting unit thatinjects a reducing agent to the catalytic unit in the exhaust pipe, aconcentration measuring unit that is arranged on a downstream side tothe catalytic unit in a flow direction of the flue gas and that measuresa concentration of nitrogen oxides in the flue gas having passed throughthe nitrogen-oxide storage-reduction catalyst, and a control unit thatcontrols whether to inject the reducing agent from the reducing-agentinjecting unit based on a concentration of nitrogen oxides measured bythe concentration measuring unit.

As described above, by measuring the concentration of nitrogen oxides bythe concentration measuring unit and controlling injection of thereducing agent based on a measurement result, the timing of performingreduction treatment can be appropriately detected. Accordingly,production of ammonia can be suppressed by the flue gas purifyingdevice, and nitrogen oxides in flue gas can be also decreased. Thenumber of times of the reduction treatment can be also decreased.

Advantageously, in the flue gas purifying device, the concentrationmeasuring unit continuously measures a concentration of nitrogenmonoxide as the concentration of nitrogen oxides. As described above, bycontinuously measuring nitrogen monoxide, the timing of performing thereduction treatment can be detected more appropriately.

Advantageously, the flue gas purifying device further includes atemperature detecting unit that detects a temperature of thenitrogen-oxide storage-reduction catalyst, the control unit storestemperature history data thereof detected by the temperature detectingunit, calculates an amount of fuel to be injected from thereducing-agent injecting unit based on the temperature history data andthe concentration of nitrogen oxides, and causes the reducing-agentinjecting unit to inject a calculated amount of fuel.

As described above, by acquiring a temperature history data, the timingof performing the reduction treatment and the amount of the reducingagent to be injected can be appropriately detected, thereby enabling tofurther decrease nitrogen oxides.

Advantageously, in the flue gas purifying device, the control unitcauses the reducing-agent injecting unit to inject the reducing agentwhen the concentration of nitrogen oxides measured by the concentrationmeasuring unit exceeds a standard value. By providing a specified valuein this manner, a reduction operation can be appropriately performed.

Advantageously, the flue gas purifying device further includes anammonia-concentration measuring unit that is arranged on a downstreamside to the catalytic unit in a flow direction of the flue gas andmeasures an ammonia concentration in the flue gas having passed throughthe nitrogen-oxide storage-reduction catalyst, and the control unitcontrols an amount of a reducing agent to be injected based on theammonia concentration measured by the ammonia-concentration measuringunit. In this manner, also by measuring the ammonia concentration, theamount of the reducing agent to be injected can be appropriatelycalculated.

Advantageously, in the flue gas purifying device, the control unitcalculates time degradation of a storage capacity of the nitrogen-oxidestorage-reduction catalyst based on the detected concentration ofnitrogen oxides and the detected ammonia concentration and controls aninjection timing and an injection amount of a reducing agent based on acalculation result thereof.

As described above, by additionally taking the storage performance ofthe nitrogen-oxide storage-reduction catalyst into consideration, theinjection timing and the injection amount of the reducing agent can becalculated more appropriately, thereby enabling to prevent production ofammonia and further decrease nitrogen oxides.

Advantageously, the flue gas purifying device further includes an SCRcatalytic unit that is arranged on a downstream side to the catalyticunit in a flow direction of the flue gas, and includes an SCR catalystthat promotes a reaction between the nitrogen oxides and ammonia, and asupport mechanism that is arranged in the exhaust pipe and supports theSCR catalyst in the exhaust pipe.

As described above, by providing the SCR catalytic unit, even if thereis ammonia leakage, ammonia is reacted with nitrogen oxides, therebyenabling to decrease or to remove ammonia.

Advantageously, the flue gas purifying device further includes apost-treatment nitrogen-oxide-concentration measuring unit that isarranged on a downstream side to the SCR catalytic unit in a flowdirection of the flue gas and measures a concentration of nitrogenoxides in flue gas having passed through the SCR catalyst, the controlunit controls injection of a reducing agent by the reducing-agentinjecting unit based on concentration of nitrogen oxides measured by thepost-treatment nitrogen-oxide-concentration measuring unit.

By providing the post-treatment nitrogen-oxide-concentration measuringunit, nitrogen oxides that cannot be treated by the SCR catalytic unitcan be detected, and by performing control based on the detection value,discharge of nitrogen oxides can be further suppressed.

Advantageously, the flue gas purifying device further includes apost-treatment ammonia-concentration measuring unit that is arranged ona downstream side to the SCR catalytic unit in a flow direction of theflue gas and measures an ammonia concentration in flue gas having passedthrough the SCR catalyst, the control unit controls injection of areducing agent by the reducing-agent injecting unit based on an ammoniaconcentration measured by the post-treatment ammonia-concentrationmeasuring unit.

By providing the post-treatment ammonia-concentration measuring unit,ammonia that cannot be treated by the SCR catalytic unit can bedetected, and by performing control based on the detection value,discharge of nitrogen oxides can be further suppressed.

ADVANTAGEOUS EFFECTS OF INVENTION

The flue gas purifying device according to the present invention canappropriately detect a timing of performing reduction treatment withrespect to a nitrogen-oxide storage-reduction catalyst, prevent nitrogenoxides and ammonia from leaking to a downstream side, and efficientlydecrease nitrogen oxides in flue gas.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram of a schematic configuration of a vehicleaccording to an embodiment of the present invention having a flue gaspurifying device according to the present invention.

FIG. 2 is a block diagram of a schematic configuration of aconcentration measuring unit in a flue gas purifying device for a dieselengine shown in FIG. 1.

FIG. 3 is a graph of an example of a relation between a NOx storagecapacity of a NOx storage catalyst and a temperature.

FIG. 4 is a graph of an example of a relation between a NOx conversionrate of a NOx storage catalyst and a temperature.

FIG. 5 is a block diagram of a schematic configuration of a vehiclehaving the flue gas purifying device according to another embodiment.

DESCRIPTION OF EMBODIMENTS

Exemplary embodiments a flue gas purifying device according to thepresent invention will be explained below in detail with reference tothe accompanying drawings. The present invention is not limited to theembodiments. In the following embodiments, it is assumed that aninternal combustion engine mounted on the flue gas purifying device is adiesel engine, and an apparatus including the internal combustion engineis a vehicle having a diesel engine. However, the internal combustionengine is not limited thereto, and the present invention is alsoapplicable to various internal combustion engines such as a gasolineengine and a gas turbine. Further, a device having the internalcombustion engine is not limited to a vehicle, and the device can beused as an internal combustion engine of various devices such as a shipand a power generator.

FIG. 1 is a block diagram of a schematic configuration of a vehicleaccording to an embodiment having a diesel engine mounted on the fluegas purifying device according to the present invention. FIG. 2 is ablock diagram of a schematic configuration of a concentration measuringunit in the flue gas purifying device shown in FIG. 1. As shown in FIG.1, a vehicle 10 includes a diesel engine 12, an exhaust pipe 14 forguiding flue gas discharged from the diesel engine 12, and a flue gaspurifying device 16 that purifies flue gas flowing in the exhaust pipe14. The vehicle 10 also includes various elements required for avehicle, such as wheels, a body, operating parts, and a transmission,other than constituent elements shown in the drawings.

The diesel engine 12 is an internal combustion engine that uses lightoil or heavy oil as a fuel, and burns the fuel to extract power. Theexhaust pipe 14 is connected to the diesel engine 12 at one end thereof,to guide flue gas discharged from the diesel engine 12.

The flue gas purifying device 16 includes an oxidation catalyst 18, afuel injecting unit 22, a fuel tank 24, a nitrogen-oxidestorage-reduction catalytic unit (hereinafter, referred as “NOx storagecatalytic unit”) 26, a concentration measuring unit 28, and a controlunit 30, and is arranged in an exhaust passage of flue gas, that is,inside the exhaust pipe 14 or adjacent to the exhaust pipe 14.

The oxidation catalyst 18 is a catalyst such as platinum provided in theexhaust passage of flue gas, specifically, inside of a downstreamportion to an exhaust port of the diesel engine 12 in a flow directionof flue gas in the exhaust pipe 14. A part of components of particulatematters (PM) in flue gas having passed in the exhaust pipe 14 andthrough the oxidation catalyst 18 is removed by the oxidation catalyst18. The PM here is an air contaminant discharged from the diesel engine,and is a mixture of solid carbon particles, unburned hydrocarbons(Soluble Organic Fraction: SOF) formed of macro-molecules, and sulfatesgenerated by oxidation of sulfur contained in the fuel. The oxidationcatalyst 18 oxidizes nitrogen monoxide contained in flue gas flowing inthe exhaust pipe 14 to nitrogen dioxide.

The fuel injecting unit 22 is an injection apparatus that injects fuelinto the exhaust pipe 14, and an injection port is provided in a portionon a downstream side to the oxidation catalyst 18 in the exhaust pipe14. The fuel injecting unit 22 injects fuel, which serves a reducingagent, into the exhaust pipe 14. from the injection port. The fuel tank24 stores and supplies fuel to the fuel injecting unit 22. Areplenishing port for replenishing fuel from an external device thatsupplies fuel is provided in the fuel tank 24, and fuel is replenishedaccording to need from the replenishing port. A fuel tank that suppliesfuel only to the fuel injecting unit 22 can be provided as the fuel tank24; however, a fuel tank that supplies fuel to the diesel engine 12 canbe used. That is, the fuel tank 24 can supply fuel to the diesel engine12 and the fuel injecting unit 22.

The NOx storage catalytic unit 26 includes a NOx storage catalyst thatstores nitrogen oxides, and a support mechanism that is provided insideof a downstream portion to the fuel injecting unit 22 in the exhaustpipe 14 and that supports the NOx storage catalyst. The NOx storagecatalytic unit 26 causes nitrogen oxides contained in flue gas havingpassed through the NOx storage catalytic unit 26 to be stored by the NOxstorage catalyst. Accordingly, nitrogen oxides in flue gas are decreasedor removed by passing through the NOx storage catalytic unit 26. Acatalyst in which an alkaline substance as a NOx storage material isadded to a conventional three-way catalyst can be used as the NOxstorage catalyst, and for example, the one in which a honeycomb carrieris coated with alumina that supports platinum (Pt), rhodium (Rh),various alkalies, alkaline earths, and rare-earth oxides can be used.Further, the support mechanism is arranged inside the exhaust pipe 14,and a hole for aerating flue gas is formed therein, and the SCR catalystis supported on the surface thereof. The support mechanism only needs tosupport the NOx storage catalyst in the exhaust pipe 14, and it can be aframe, for example.

The concentration measuring unit 28 is arranged in the exhaust pipe 14on the downstream side of the NOx storage catalytic unit 26 in theexhaust passage of flue gas and measures the concentration of nitrogenoxides in flue gas having passed through the NOx storage catalytic unit26. As shown in FIG. 2, the concentration measuring unit 28 includes ameasuring unit body 40, an optical fiber 42, a measuring cell 44, and alight receiving unit 46. In the present embodiment, a case that theconcentration of nitrogen monoxide in nitrogen oxides is measured by theconcentration measuring unit 28 is explained.

The measuring unit body 40 has a light emitting unit that emits laserbeams in a wavelength region absorbed by nitrogen monoxide, and acomputing unit that calculates the nitrogen monoxide concentration froma signal. The measuring unit body 40 outputs laser beams to the opticalfiber 42 and receives a signal received by the light receiving unit 46.

The optical fiber 42 guides laser beams output from the measuring unitbody 40 so as to enter into the measuring cell 44.

The measuring cell 44 is arranged in a part of the exhaust pipe 14, andincludes an incident unit that causes light emitted from the opticalfiber 42 to enter into the measuring cell 44, and an output unit thatoutputs laser beams having passed through a predetermined path in themeasuring cell 44.

The light receiving unit 46 receives laser beams having passed throughthe inside of the measuring cell 44 and output from the output unit, andoutputs an intensity of received laser beams to the measuring unit body40 as a light receiving signal.

The configuration of the concentration measuring unit 28 is as describedabove, and laser beams output from the measuring unit body 40 passthrough the predetermined path in the measuring cell 44 from the opticalfiber 42 and is output from the output unit. At this time, if nitrogenmonoxide is contained in flue gas in the measuring cell 44, laser beamspassing through the measuring cell 44 are absorbed. Therefore, an outputof laser beams reaching the output unit changes according to the carbonmonoxide concentration in flue gas. The light receiving unit 46 convertslaser beams output from the output unit to a light receiving signal, andoutputs the light receiving signal to the measuring unit body 40. Themeasuring unit body 40 compares the intensity of output laser beams withan intensity calculated from the light receiving signal, to calculatethe nitrogen monoxide concentration in flue gas flowing in the measuringcell 44 based on its rate of diminution. Thus, the concentrationmeasuring unit 28 uses TDLAS (Tunable Diode Laser AbsorptionSpectroscopy) to calculate or measure the nitrogen monoxideconcentration in flue gas passing through a predetermined position inthe measuring cell 44, that is, a measurement position based on theintensity of output laser beams and the light receiving signal detectedby the light receiving unit 46. The concentration measuring unit 28according to the present embodiment can continuously calculate ormeasure the nitrogen monoxide concentration.

Only the incident unit and the output unit of the measuring cell 44 canbe made of a light transmitting material, or the measuring cell 44 onthe whole can be made of the light transmitting material. Further, atleast two optical mirrors can be provided in the measuring cell 44, sothat laser beams entering from the incident unit is multiply-reflectedby the optical mirrors and output from the output unit. Bymultiply-reflecting laser beams, laser beams can pass through moreregions in the measuring cell 44. Accordingly, an influence ofconcentration distribution on flue gas flowing in the measuring cell 44can be decreased, thereby enabling accurate detection of concentrations.

The control unit 30 controls the injection timing and the injectionamount of fuel to be injected from the fuel injecting unit 22 accordingto PID control based on a detection result obtained by the concentrationmeasuring unit 28. Specifically, when the concentration of nitrogenmonoxide becomes higher than a reference value, the control unit 30determines that more than a certain amount of NOx is stored in the NOxstorage catalytic unit 26 and the NOx storage catalytic unit 26 cannotstore NOx any more, and causes the fuel injecting unit 22 to injectfuel. Further, the control unit 30 controls the injection amount basedon a change rate of the concentration of nitrogen monoxide and aprevious operating condition. The configuration of the vehicle 10 is asdescribed above.

When the diesel engine 12 is driven, the vehicle 10 discharges flue gascontaining nitrogen oxides. The flue gas discharged from the dieselengine 12 passes through the exhaust pipe 14 and reaches the oxidationcatalyst 18. In the flue gas having reached the oxidation catalyst 18,nitrogen monoxide contained in flue gas is oxidized by the oxidationcatalyst 18 to become nitrogen dioxide. The flue gas oxidized by theoxidation catalyst 18 further passes through the exhaust pipe 14 andreaches the NOx storage catalytic unit 26. In the flue gas havingreached the NOx storage catalytic unit 26, nitrogen oxides contained inthe flue gas are stored by the NOx storage catalytic unit 26. The fluegas in which nitrogen oxides contained in the flue gas are decreased andremoved by the NOx storage catalytic unit 26 further passes through theexhaust pipe 14, and is discharged outside after the concentration ofnitrogen monoxide is measured by the concentration measuring unit 28.The concentration measuring unit 28 transmits a concentrationmeasurement result of nitrogen monoxide in flue gas to the control unit30.

In this manner, the NOx storage catalytic unit 26 continuously storesnitrogen oxides contained in flue gas passing therethrough. There is alimit to the nitrogen oxide (NOx) storage capacity of the NOx storagecatalyst in the NOx storage catalytic unit 26, and when a certain amountof nitrogen oxides (exceeding the limit to the storage capacity) isstored, the NOx storage catalyst cannot store nitrogen oxides any more.Therefore, when the nitrogen oxide storage capacity of the NOx storagecatalyst exceeds the limit, the concentration of nitrogen oxides in fluegas, which passes through the NOx storage catalytic unit 26 and isdischarged to outside, increases.

The control unit 30 determines whether the storage capacity of the NOxstorage catalyst in the NOx storage catalytic unit 26 has reached athreshold limit based on the concentration of nitrogen monoxide measuredby the concentration measuring unit 28. When determining that thestorage capacity has reached the threshold limit, the control unit 30performs the reduction treatment of the NOx storage catalyst. Thecontrol unit 30 can determine whether the storage capacity has reachedthe threshold limit by determining whether the concentration of nitrogenmonoxide exceeds the reference value. The reduction treatment isexplained below. The control unit 30 injects fuel from the fuelinjecting unit 22 into the exhaust pipe 14. Fuel injected into theexhaust pipe 14 is partially oxidized in the exhaust pipe 14 to producecarbon monoxide, hydrogen, and carbon hydride. Produced carbon monoxide,hydrogen, and carbon hydride react with nitrogen oxides (NO, NO₂) storedin the NOx storage catalyst. Carbon monoxide (CO) becomes carbon dioxide(CO₂), hydrogen (H₂) becomes water (H₂O), carbon hydride becomes water(H₂O) and carbon dioxide (CO₂), and nitrogen oxides become nitrogen(N₂), thereby removing nitrogen oxides from the NOx storage catalyst.

In this manner, the vehicle 10 and the flue gas purifying device 16perform the reduction treatment of the NOx storage catalyst at thetiming determined based on the concentration of nitrogen monoxidemeasured by the concentration measuring unit 28, for example, when theconcentration of nitrogen monoxide becomes higher than the referencevalue, thereby enabling to remove nitrogen oxides stored by the NOxstorage catalyst, so that the NOx storage catalytic unit 26 can storenitrogen oxides in flue gas again. The vehicle 10 and the flue gaspurifying device 16 can optimally store nitrogen oxides contained influe gas in the NOx storage catalyst by regularly performing thereduction treatment in this manner. That is, it can be prevented thatthe NOx storage catalyst cannot store nitrogen oxides contained in fluegas any more.

In the present embodiment, the timing of performing the reductiontreatment is determined based on the concentration of nitrogen monoxidemeasured by the concentration measuring unit 28, thereby enabling toperform the reduction treatment of the NOx storage catalyst at anappropriate timing, even if the operating condition is complicated, thecapacity of the NOx storage catalyst deteriorates, or the temperaturerapidly changes.

FIG. 3 is a graph of an example of a relation between the NOx storagecapacity of the NOx storage catalyst and a temperature. For example, asshown in FIG. 3, the nitrogen oxide storage capacity of the NOx storagecatalyst largely changes according to the temperature. However, it isdifficult to accurately detect the temperature and appropriatelycalculate the threshold limit of the storage capacity. Therefore, whenthe nitrogen oxide storage capacity and the threshold limit are to bedetermined based on the data map calculated beforehand, it is requiredto set the threshold limit lower than the actual threshold limit so asnot to exceed the threshold limit. That is, it is required to set thethreshold limit lower than the actual threshold limit for suppressingleakage of nitrogen oxides, even if the threshold limit of the storagecapacity drops due to a decrease in capacity of the NOx storage catalystor a deviation from an appropriate temperature. Because it is difficultto accurately calculate a storage rate of NOx or a produced amount ofNOx, the calculated storage capacity may be different from the actualstorage capacity. When the deviation increases, fuel (as a reducingagent), may be excessively injected, so that the reduction treatmentexcessively proceeds to produce ammonia (NH₃). On the other hand,because the vehicle 10 and the flue gas purifying device 16 detect thereduction timing based on the result measured by the concentrationmeasuring unit 28, the threshold limit of the storage capacity can beaccurately detected, and the storage capacity of the NOx storagecatalyst can be appropriately recovered, regardless of the operatingcondition, performance of NOx storage catalyst, and temperatureenvironment. Because the threshold limit of the storage capacity can beaccurately ascertained, even if a larger amount of fuel is injected atthe time of the reduction treatment, stored nitrogen oxides can bereliably reacted with the fuel. Accordingly, a larger amount of nitrogenoxides can be reduced while suppressing production of ammonia.Therefore, the number of times of reduction treatment to be performedcan be further decreased.

Because the threshold limit of the storage capacity can be accuratelyascertained, the injection amount of fuel can be calculated moreaccurately, thereby enabling to suppress that the fuel is injected toomuch and reduction of nitrogen oxides excessively proceeds to produceammonia. The timing of the reduction treatment can be controlled basedon only the concentration of nitrogen oxides, and only one sensor needsto be provided, thereby enabling to simplify a device configuration.

It is desired that the flue gas purifying device performs the reductiontreatment while controlling the temperature. The temperature can becontrolled by using a temperature adjusting mechanism such as a heateror a cooling fan. FIG. 4 is a graph of an example of a relation betweena NOx conversion ratio of the NOx storage catalyst and a temperature. Asshown in FIG. 4, the NOx conversion ratio of the NOx storage catalystchanges according to the temperature. Therefore, the reduction treatmentis performed while controlling the temperature, thereby enabling toreduce the NOx storage catalyst efficiently. The supplied fuel can beefficiently used by stabilizing the temperature, and unreactedsubstances can be decreased.

Also, the control unit 30 can calculate an approximate value of thetiming of performing the reduction treatment or an approximate value ofthe injection amount of fuel based on the operating conditions such asaccelerator opening, velocity, engine speed, and temperature.Specifically, the produced amount of nitrogen monoxide can be calculatedbased on the operating conditions by using the data map showing arelation between the operating conditions and an emission amount ofnitrogen monoxide, to calculate the estimate of the timing of performingthe reduction treatment or the estimate of the injection amount of fuel.The timing of performing the reduction treatment can be easilycalculated by calculating the estimate of the timing according to theoperating conditions. It can be prevented to perform the reductiontreatment when the concentration of nitrogen oxides irregularly risesonly for a moment. The calculation accuracy of the injection amount offuel can be further increased by calculating the estimate of theinjection amount of fuel. That is, more accurate injection amount can becalculated. Accordingly, excessive injection of fuel can be suppressedmore reliably.

When the approximate value is not to be calculated, the operatingconditions do not need to be detected, thereby decreasing the number ofmeasuring units, and enabling to simplify the device configuration ofthe flue gas purifying device. Because the reference value does not needto be calculated according to the condition, the control can besimplified.

It is desired to provide a filter for trapping particulate matterscontained in flue gas in the flue gas purifying device 16. Particulatematters (PM) contained in flue gas can be trapped by providing thefilter that traps particulate matters in addition to the oxidationcatalyst. It is desired to provide the oxidation catalyst as in thepresent embodiment, because particulate matters can be trapped, andnitrogen oxides are unified to nitrogen dioxide so as to be easilytreated. However, the oxidation catalyst does not need to be provided.

In the flue gas purifying device 16, the concentration measuring unit 28can measure nitrogen monoxide continuously without detecting nitrogenoxides. Therefore, the concentration measuring unit 28 uses the TDLASmethod in which laser beams in a wavelength region absorbed by nitrogenmonoxide are output and an absorption rate of laser beams is detected,to measure the concentration of nitrogen monoxide. However, the presentinvention is not limited thereto. Various measuring units that canmeasure the concentration of nitrogen monoxide in flue gas can be usedin the present invention. For example, a branch pipe can be provided ata measurement position, so that a part of flue gas is caused to flowinto the branch pipe to measure the concentration of nitrogen monoxidein flue gas flowing in the branch pipe. The concentration of nitrogenmonoxide can be accurately measured by using the TDLAS method, withoutcausing noise due to ammonia or the like.

Only nitrogen monoxide is detected as nitrogen oxides to be detected bythe concentration measuring unit 28. However, the present invention isnot limited thereto, and only nitrogen dioxide can be detected ornitrogen monoxide and nitrogen dioxide can be both detected. Even whenonly one of nitrogen monoxide and nitrogen dioxide is detected ornitrogen monoxide and nitrogen dioxide are both detected, theconcentration of nitrogen oxides in flue gas can be suitably measured,and the timing of performing the reduction treatment can be calculatedbased on a measurement. When nitrogen monoxide and nitrogen dioxide areboth detected according to the TDLAS method by using the concentrationmeasuring unit 28, two sensors can be provided to perform measurement,or for example, the concentration of nitrogen monoxide and nitrogendioxide by emitting light having two wavelength regions by one sensorcan be measured with the one sensor.

In the flue gas purifying device 16, only the concentration measuringunit 28 is provided to control the fuel injecting unit 22 based on theconcentration of nitrogen oxides (nitrogen monoxide) in flue gas havingpassed through the NOx storage catalytic unit 26. However, the presentinvention is not limited thereto. A flue gas purifying device accordingto another embodiment of the present invention is explained below withreference to FIG. 5.

FIG. 5 is a block diagram of a schematic configuration of a vehicleaccording to another embodiment having the flue gas purifying device. Avehicle 50 shown in FIG. 5 has the same configuration as that of thevehicle 10, except of a part of a flue gas purifying device 52, andtherefore explanations of like constituent elements will be omitted, andfeatures specific to the vehicle 50 are mainly explained below. Thevehicle 50 shown in FIG. 5 includes the diesel engine 12, the exhaustpipe 14, and the flue gas purifying device 52. The flue gas purifyingdevice 52 includes the oxidation catalyst 18, the fuel injecting unit22, the fuel tank 24, the NOx storage catalytic unit 26, theconcentration measuring unit 28, an SCR catalytic unit 54, anammonia-concentration measuring unit 56, a post-treatmentnitrogen-oxide-concentration measuring unit 59, a post-treatmentammonia-concentration measuring unit 60, and a control unit 62. Theoxidation catalyst 18, the fuel injecting unit 22, the fuel tank 24, theNOx storage catalytic unit 26, and the concentration measuring unit 28respectively have the same configuration as those of the flue gaspurifying device 16 described above, and thus detailed explanationsthereof will be omitted. The concentration measuring unit 28 is arrangedbetween the NOx storage catalytic unit 26 and the SCR catalytic unit 54.

The SCR catalytic unit 54 includes an SCR catalyst, which is a ureaselective reduction catalyst; that traps ammonia and promotes a reactionof ammonia with nitrogen oxides, and a support mechanism provided insidethe exhaust pipe 14 in a portion on a downstream side to the NOx storagecatalytic unit 26 to support the SCR catalyst. A zeolitic catalyst canbe used as the SCR catalyst. A hole arranged in the exhaust pipe 14 foraerating flue gas is formed in the support mechanism, and the SCRcatalyst is supported on the surface thereof. The support mechanism canbe, for example, a frame so long as it can support the SCR catalyst inthe exhaust pipe 14.

The configuration of the SCR catalytic unit 54 is described above, andflue gas with nitrogen oxides being stored in the NOx storage catalyticunit 26 is supplied to the SCR catalytic unit 54 via the exhaust pipe14. When ammonia is contained in flue gas to be supplied, the SCRcatalytic unit 54 traps ammonia contained in flue gas. Ammonia is mixedin flue gas produced when the NOx storage catalytic unit 26over-supplies fuel. When nitrogen oxides remain in flue gas suppliedfrom the exhaust pipe 14, the SCR catalytic unit 54 causes nitrogenoxides contained in flue gas to react with trapped ammonia, so thatoxygen is removed from nitrogen oxides and nitrogen oxides are reducedto nitrogen. Specifically, nitrogen oxides are reduced according to thefollowing chemical reaction.

4NH₃+4NO+O₂→4N₂+6H₂O

4NH₃+2NO₂+O₂→3N₂+6H₂O

The ammonia-concentration measuring unit 56 is arranged in the exhaustpipe 14 on the downstream side of the NOx storage catalytic unit 26 andon an upstream side of the SCR catalytic unit 54 in the exhaust passageof flue gas and measures the concentration of ammonia in flue gasdischarged from the NOx storage catalytic unit 26. Theammonia-concentration measuring unit 56 includes a measuring unit body,an optical fiber, a measuring cell, and a light receiving unit as in theconcentration measuring unit 28. The measuring method of the ammoniaconcentration by the ammonia-concentration measuring unit 56 is the sameas that of the concentration, measuring unit 28, and thus explanationsthereof will be omitted. The ammonia-concentration measuring unit 56continuously measures the concentration of ammonia contained in flue gashaving passed through the NOx storage catalytic unit 26 and transmits ameasurement result to the control unit 62.

The post-treatment nitrogen-oxide-concentration measuring unit 58 isarranged in the exhaust pipe 14 on the downstream side of the SCRcatalytic unit 54 in the exhaust passage of flue gas and measures theconcentration of nitrogen oxides in flue gas having passed through theSCR catalytic unit 54. A sensor having the same configuration as that ofthe concentration measuring unit 28 can be used as the post-treatmentnitrogen-oxide-concentration measuring unit 58. The post-treatmentnitrogen-oxide-concentration measuring unit 58 continuously measures theconcentration of nitrogen oxides contained in flue gas having passedthrough the SCR catalytic unit 54 and transmits a measurement result tothe control unit 62.

The post-treatment ammonia-concentration measuring unit 60 is arrangedin the exhaust pipe 14 on the downstream side of the SCR catalytic unit54 in the exhaust passage of flue gas and measures the ammoniaconcentration in flue gas having passed through the SCR catalytic unit54. A sensor having the same configuration as that of theammonia-concentration measuring unit 56 can be used as thepost-treatment ammonia-concentration measuring unit 60. Thepost-treatment ammonia-concentration measuring unit 60 continuouslymeasures the concentration of ammonia contained in flue gas havingpassed through the SCR catalytic unit 54 and transmits a measurementresult to the control unit 62.

The control unit 62 controls an injection timing of fuel by the fuelinjecting unit 22, that is, a timing of the reduction treatment and anamount of fuel to be injected based on the measurement resultstransmitted from the concentration measuring unit 28, theammonia-concentration measuring unit 56, the post-treatmentnitrogen-oxide-concentration measuring unit 58, and the post-treatmentammonia-concentration measuring unit 60. The configurations of thevehicle 50 and the flue gas purifying device 52 are basically asdescribed above.

The control unit 62 basically calculates the timing of the reductiontreatment and the amount of fuel to be injected based on a measurementof the concentration measuring unit 28. When the amount of fuel to beinjected to nitrogen oxides stored by the NOx storage catalystincreases, ammonia is produced. However, the production of ammonia canbe detected by measuring the concentration of ammonia contained in fluegas by the ammonia-concentration measuring unit 56. When ammonia ismeasured, the control unit 62 determines that the injection amount offuel is excessive, and decreases the injection amount of fuel. In thisway, by measuring the ammonia concentration by the ammonia-concentrationmeasuring unit 56, a decrease of the storage capacity due to a decreaseof capacity of the NOx storage catalytic unit 26 can be detected,thereby enabling to control the injection amount of fuel accurately, andsuppress discharge of ammonia due to oversupply of fuel.

Further, by providing the SCR catalytic unit 54, the SCR catalytic unit54 can trap ammonia even when ammonia is produced due to the NOx storagecatalytic unit 26, thereby enabling to suppress discharge of ammonia tooutside of the flue gas purifying device.

It is desired that the control unit 62 calculates a performance shift ofthe NOx storage catalyst in the NOx storage catalytic unit 26 based onthe measurement results obtained by the concentration measuring unit 28and the ammonia-concentration measuring unit 56. That is, the controlunit 62 calculates the amount of stored nitrogen oxides based on thetiming of performing the reduction treatment, the amount of ammonia tobe discharged at the time of reduction and the like, thereby tocalculate the performance shift of the NOx storage catalyst, forexample, the degree of time degradation. In this manner, by calculatingthe performance shift of the NOx storage catalyst, an appropriate timingof performing the reduction treatment and the injection amount of fuelcan be calculated.

When the ammonia-concentration measuring unit 56 detects that ammonia isdischarged from the NOx storage catalytic unit 26, the control unit 62calculates an accumulated amount of discharged ammonia, and controls thetiming of the reduction treatment so that nitrogen oxides correspondingto a discharge amount of ammonia are discharged from the NOx storagecatalytic unit 26. Specifically, the control unit 62 delays the timingof the reduction treatment so that the reduction treatment is notperformed for a certain time even if the threshold limit of the NOxstorage catalytic unit 26 is exceeded. Accordingly, nitrogen oxidescontained in flue gas are not stored in the NOx storage catalytic unit26, but are supplied to the SCR catalytic unit 54, so that nitrogenoxides can react with ammonia trapped by the SCR catalytic unit 54,thereby enabling to remove ammonia. Accordingly, discharge of ammoniaand nitrogen oxides from the flue gas purifying device can be furthersuppressed.

When the post-treatment nitrogen-oxide-concentration measuring unit 58detects nitrogen oxides contained in flue gas, the control unit 62causes the fuel injecting unit 22 to inject more fuel. It can bedetected whether ammonia is trapped by the SCR catalytic unit 54 bydetecting nitrogen oxides in flue gas by the post-treatmentnitrogen-oxide-concentration measuring unit 58. When the post-treatmentnitrogen-oxide-concentration measuring unit 58 detects that nitrogenoxides are contained in flue gas, the control unit 62 causes the fuelinjecting unit 22 to inject more fuel, thereby bringing a state wherethe SCR catalytic unit 54 traps ammonia. In this manner, by achievingthe state where the SCR catalytic unit 54 traps ammonia, even if the NOxstorage catalytic unit 26 cannot trap nitrogen oxides, the SCR catalyticunit 54 can trap nitrogen oxides, and discharge of nitrogen oxides tooutside from the flue gas purifying device 52 can be suppressed.

Furthermore, when the post-treatment ammonia-concentration measuringunit 60 detects that ammonia is contained in flue gas, the control unit62 delays the timing of the reduction treatment for a certain time sothat nitrogen oxides are discharged from the NOx storage catalytic unit26. It can be determined whether the SCR catalytic unit 54 traps ammoniamore than the limit by detecting ammonia by the post-treatmentammonia-concentration measuring unit 60. When the post-treatmentammonia-concentration measuring unit 60 detects that ammonia iscontained in flue gas, the control unit 62 delays the timing of thereduction treatment to cause the NOx storage catalytic unit 26 todischarge a certain amount of nitrogen oxides. Accordingly, ammoniatrapped in the SCR catalytic unit 54 can be reduced, thereby enabling tosuppress discharge of ammonia from the SCR catalytic unit 54.

The SCR catalytic unit 54, the ammonia-concentration measuring unit 56,the post-treatment nitrogen-oxide-concentration measuring unit 58, andthe post-treatment ammonia-concentration measuring unit 60 are providedin the vehicle 50 and the flue gas purifying device 52, because variouseffects can be achieved. However, certain effects can be achieved onlyby partly providing a configuration of these units. The post-treatmentnitrogen-oxide-concentration measuring unit 58 and the post-treatmentammonia-concentration measuring unit 60 can achieve the effectsdescribed above when the SCR catalytic unit 54 is provided.

It is desired that the flue gas purifying device 52 is provided with atemperature detecting unit that detects the temperature of the NOxstorage catalyst to control the timing of performing the reductiontreatment and the injection amount of fuel also by using the temperatureof the NOx storage catalyst detected by the temperature detecting unit.Because the performance of the NOx storage catalyst changes according tothe temperature, the temperature is detected and the injection amount iscalculated by using a temperature history thereof, thereby enabling tocalculate the more appropriate injection amount, and achieve a requisiteminimum injection amount, Accordingly, appropriate control can beperformed, while suppressing discharge of ammonia and nitrogen oxidesfrom the flue gas purifying device.

In the above embodiments, it has been explained that the flue gaspurifying device includes the concentration measuring unit 28 thatmeasures the concentration of nitrogen oxides. However, only theammonia-concentration measuring unit 56 in FIG. 5 can be providedwithout providing the concentration measuring unit 28. In this manner,when only the ammonia-concentration measuring unit 56 is provided, anappropriate timing of the reduction treatment (that is, fuel injection)cannot be calculated. However, excessive injection of fuel can besuppressed and discharge of ammonia can be suppressed.

Any type of fuel such as methanol, propylene, kerosene, and light gas,oil can be used as the fuel to be injected from the fuel injecting unit,so long as it is a liquid that can reduce nitrogen oxides. Further, itis not limited to the fuel, and any reducing agent that reduces nitrogenoxides can be used.

INDUSTRIAL APPLICABILITY

As described above, the flue gas purifying device according to thepresent invention is useful for purifying flue gas discharged from aninternal combustion engine, and the flue gas purifying device isparticularly suitable for purifying flue gas discharged from a dieselengine mounted on a vehicle.

REFERENCE SIGNS LIST

-   10, 50 vehicle-   12 diesel engine-   14 exhaust pipe-   16, 52 flue gas purifying device-   18 oxidation catalyst-   22 fuel injecting unit-   24 fuel tank-   26 NOx storage catalytic unit-   28 concentration measuring unit-   30, 62 control unit-   40 measuring unit body-   42 optical fiber-   44 measuring cell-   46 light receiving unit-   54 SCR catalytic unit-   56 ammonia-concentration measuring unit-   58 post-treatment nitrogen-oxide-concentration measuring unit-   60 post-treatment ammonia-concentration measuring unit

1. A flue gas purifying device that reduces nitrogen oxides contained influe gas discharged from an internal combustion engine, the devicecomprising: an exhaust pipe that guides flue gas discharged from theinternal combustion engine; a catalytic unit that is arranged on adownstream side to the internal combustion engine in a flow direction ofthe flue gas and includes a nitrogen-oxide storage-reduction catalystthat stores nitrogen oxides contained in the flue gas and a supportmechanism that is arranged in the exhaust pipe and supports thenitrogen-oxide storage-reduction catalyst in the exhaust pipe; areducing-agent injecting unit that injects a reducing agent to thecatalytic unit in the exhaust pipe; a concentration measuring unit thatis arranged on a downstream side to the catalytic unit in a flowdirection of the flue gas and measures a concentration of nitrogenoxides in the flue gas having passed through the nitrogen-oxidestorage-reduction catalyst; and a control unit that controls whether toinject the reducing agent from the reducing-agent injecting unit basedon a concentration of nitrogen oxides measured by the concentrationmeasuring unit.
 2. The flue gas purifying device according to claim 1,wherein the concentration measuring unit continuously measures aconcentration of nitrogen monoxide as the concentration of nitrogenoxides.
 3. The flue gas purifying device according to claim 1, furthercomprising a temperature detecting unit that detects a temperature ofthe nitrogen-oxide storage-reduction catalyst, wherein the control unitstores temperature history data thereof detected by the temperaturedetecting unit, calculates an amount of fuel to be injected from thereducing-agent injecting unit based on the temperature history data andthe concentration of nitrogen oxides, and causes the reducing-agentinjecting unit to inject a calculated amount of fuel.
 4. The flue gaspurifying device according to claim 1, wherein the control unit causesthe reducing-agent injecting unit to inject the reducing agent when theconcentration of nitrogen oxides measured by the concentration measuringunit exceeds a standard value.
 5. The flue gas purifying deviceaccording to claim 1, further comprising an ammonia-concentrationmeasuring unit that is arranged on a downstream side to the catalyticunit in a flow direction of the flue gas and measures an ammoniaconcentration in the flue gas having passed through the nitrogen-oxidestorage-reduction catalyst, and the control unit controls an amount of areducing agent to be injected based on the ammonia concentrationmeasured by the ammonia-concentration measuring unit.
 6. The flue gaspurifying device according to claim 5, wherein the control unitcalculates time degradation of a storage capacity of the nitrogen-oxidestorage-reduction catalyst based on the detected concentration ofnitrogen oxides and the detected ammonia concentration and controls aninjection timing and an injection amount of a reducing agent based on acalculation result thereof.
 7. The flue gas purifying device accordingto claim 1, further comprising an SCR catalytic unit that is arranged ona downstream side to the catalytic unit in a flow direction of the fluegas, and includes an SCR catalyst that promotes a reaction between thenitrogen oxides and ammonia, and a support mechanism that is arranged inthe exhaust pipe and supports the SCR catalyst in the exhaust pipe. 8.The flue gas purifying device according to claim 7, further comprising apost-treatment nitrogen-oxide-concentration measuring unit that isarranged on a downstream side to the SCR catalytic unit in a flowdirection of the flue gas and measures a concentration of nitrogenoxides in flue gas having passed through the SCR catalyst, wherein thecontrol unit controls injection of a reducing agent by thereducing-agent injecting unit also based on concentration of nitrogenoxides measured by the post-treatment nitrogen-oxide-concentrationmeasuring unit.
 9. The flue gas purifying device according to claim 7,further comprising a post-treatment ammonia-concentration measuring unitthat is arranged on a downstream side to the SCR catalytic unit in aflow direction of the flue gas and measures an ammonia concentration influe gas having passed through the SCR catalyst, wherein the controlunit controls injection of a reducing agent by the reducing-agentinjecting unit also based on an ammonia concentration measured by thepost-treatment ammonia-concentration measuring unit.